The present invention relates to radio communication systems. In particular, the present invention is related to communication systems which use frequency hopping in unlicensed frequency carriers.
In the last decades, progress in radio and VLSI technology has fostered widespread use of radio communications in consumer applications, portable devices, such as mobile radios, can now be produced having acceptable cost, size and power consumption.
Although wireless technology is today focused mainly on voice communications (e.g. with respect to handheld radios), this field will likely expand in the near future to provide greater information flow to and from other types of nomadic devices and fixed devices. More specifically, it is likely that further advances in technology will provide very inexpensive radio equipment which can be easily integrated into many devices. This will reduce the number of cables currently used. For instance, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals.
The aforementioned radio communications will require an unlicensed band with sufficient capacity to allow for high data rate transmissions. A suitable band is the ISM (Industrial, Scientific and Medical) band at 2.4 GHz, which is globally available. The ISM band provides about 83.5 MHZ of radio spectrum.
To allow different radio networks to share the same radio medium without coordination, signal spreading is usually applied. In fact, the FCC in the United States currently requires radio equipment operating in the 2.4 GHz band to apply some form of spreading when the transmit power exceeds about 0 dBm. Spreading can either be at the symbol level by applying direct-sequence (DS) spread spectrum or at the channel level by applying frequency hopping (FH) spread spectrum. The latter is attractive for the radio applications mentioned above since it more readily allows the use of cost-effective radios. A system called Bluetooth was recently introduced to provide pervasive connectivity especially between portable devices like mobile phones, laptops, PDAs, and other nomadic devices. This system applies FH to enable the construction of low-power, low-cost radios with a small footprint. The system supports both data and voice, the latter being optimized by applying fast FH with a nominal rate of 800 hops/s through the entire ISM band in combination with a robust voice coding. The system concept includes piconets consisting of a master and a limited number of slaves sharing the same 1 MHZ channel. The system also features low-power modes like HOLD and PARK where the slaves can be put in a temporary suspend or low duty cycle tracking mode, respectively. For additional information regarding the Bluetooth system, see xe2x80x9cBluetooth, the Universal Radio Interface for Ad Hoc wireless connectivityxe2x80x9d, J. C. Haartsen, Ericsson Review, Telecommunications Technology Journal, No. 3, 1998.
In an FH system deploying transmit power above 0 dBm, the channel bandwidth may be limited to 1 MHZ. Limiting bandwidth correspondingly restricts data rates to the 1-2 Mb/s range. However, especially for data services like file transfer or file download, ever-increasing data rates are desirable. In a FH system with a limited hop bandwidth (e.g. 1 MHZ), high data rates are difficult to obtain. In a DS system, high data rates are also difficult to obtain at reasonable costs. DS systems have the additional disadvantage of the near-far problem which becomes more serious in uncoordinated scenarios for which the Bluetooth system was optimized. In scenarios where a Bluetooth system is used, communications over short distances (e.g. cable replacement applications) is common practice. In these applications, a data rate in excess of 2 Mb/s would be highly desirable. Yet, by its nature, the system must operate in unlicensed bands where interference cannot be controlled.
It would therefore be appreciated that a need exists in the art for a method and apparatus for providing low-rate, medium-rate, and high-rate data communications concurrently between communications entities over the same unlicensed frequency carrier.
It is therefore an object of the present invention to provide a communications system for conducting low, medium and high rate communications over a shared communications channel.
It is a further object of the present invention to provide such a communications system having a narrow and wide band communication link over the same shared communication channel.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in a method and apparatus which applies a narrow band FH link for low-rate and medium-rate communications, and a stationary wide band link for high-speed (HS) communications. The system, generally, may include a master and one or more slaves which all share the same FH link. The master and slaves may form a piconet. Master and slaves may hop synchronously according to a pseudo-random hop sequence. The sequence may be determined by the master identity, the phase in the sequence may be determined by the master real-time system clock. The master may control the traffic on the link. An HS link can be established between the master and one or more slaves or between two slaves.
The high-speed link need not make use of a hopping scheme, and instead an appropriate band of the radio spectrum may be selected to establish the HS link. The selection is based on RSSI measurements both in master and slave, preferably carried out during the low-rate communications in the piconet. The HS link may be placed on the radio band carrying, on average, the lowest amount of interference. The selection is adaptive in the sense that the system avoids using a radio band with much interference for the HS link. If the master is involved in communications with one or more slaves over an HS link, the master has to share its time between the HS slave on the HS link and the slaves remaining on the FH link. Time division multiplexing may be applied where the master, during a certain time interval, resides on an HS link and during the remaining time resides on the FH link. If the master is not involved in communications over an HS link, e.g. two slaves establish an HS link, then piconet communications over an FH link may progress in parallel with the HS link. If the portion of the shared radio band which the HS link uses, is part of the band over which the piconet hops, the master may control traffic such that the HS link is never visited by the FH link. If the HS link and the FH link do not overlap, then such hop avoidance is not required.
The HS slave-pair may further remain in contact with the master by a beacon signal which may be used on the FH link. Periodically, HS slaves may interrupt their HS communications and temporarily listen to the master on the FH link. This beacon also provides a means for the slaves to return from the HS link to the FH link. In an alternative embodiment, the two slaves are directed to the HS link for a limited amount of time. After the time interval has expired, slaves engaged in communications over the HS link may automatically return to the FH link. If required, slaves may be sent to the HS link again. If the HS link experiences interference, the units participating in the HS link return to the FH link and a new HS link can be negotiated. New RSSI measurements will show where the HS link can best be placed.